There's no consensus in terminology, so please consider my answer as "one of many".
Résumé is usually a static document which changes along with your career.
Cover letter should be individual per application. Its primary intent is to tell the HR why you are eligible (and the best!) candidate for the very position you are applying to, by polite highlighting only those of your expertises that worth more for a particular position.
There are opinions the HR would never read cover letters. It is correct, because people write bad cover letters! The main goal of cover letter is to save HR's time. If you are not someone they are looking for, cover letter will provide with opportunity to quickly skip your résumé. There's nothing bad in skipping. You express kindness toward the HR, and you should expect kindness from them by keeping your résumé atop of the others for future use.
If you could link to your own personal successful cover letter that would be best
A small sample.
Your CV tells jobs and projects you've been working for, you don't change it very often.
Say, you're applying to a
Project Manager position in a company that makes
XML-based tools for
e-Learning industry, and you know their soft is written in
.NET. Your cover letter may contain something like:
• My strongest technology is .NET (C#, ASP.NET) - X years;
• X years experience with XML, XSLT, XSL-FO and related technologies;
• ... interest in languages (X years Chinese, X years Spanish) and theoretical linguistics;
... results across entire development cycle:
• ... initial research and prototyping;
• ... key business requirements ...;
• ... project architecture ...;
• ... automated team collaboration procedures and TDD practices; Risk Management;
• ... development on time and on budget ...;
• ... communications with the customers;
• ... deployment and support ...;